A disk drive system includes one or more magnetic recording disks and control mechanisms for storing data within approximately circular tracks on the disk. A disk is composed of a substrate and one or more layers deposited on the substrate (e.g., aluminum). A sheet of the substrate material may be punched or scribed along outer and inner diameter contours to generate the substrate having an inner diameter (ID) and an outer diameter (OD).
A trend in the design of disk drive systems is to increase the recording density of the magnetic recording disk used in the system. One method for increasing recording density is to pattern the surface of the disk with discrete tracks, referred to as discrete track recording (DTR). A DTR pattern may be formed by nano-imprint lithography (NIL) techniques, in which a pre-embossed forming tool (a.k.a., stamper, embosser, etc.), having an inverse pattern to be imprinted, is pressed into an embossable film (i.e., polymer) disposed above a disk substrate to form an initial pattern of compressed areas. This initial pattern ultimately forms a pattern of raised and recessed areas. After stamping the embossable film, an etching process may be used to transfer the pattern through the embossable film by removing the residual film in the compressed areas. After the imprint lithography process, another etching process may be used to form the pattern in a base structure having one or more layers (e.g., substrate, nickel-phosphorous, soft magnetic layer, etc.) residing underneath the embossable film. The resulting DTR track structure contains a pattern of concentric raised areas and recessed areas under a magnetic recording layer. The raised areas (also known as hills, lands, elevations, etc.) are used for storing data and the recessed areas (also known as troughs, valleys, grooves, etc.) that may store servo (head positioning) information and provide inter-track isolation to reduce noise. The servo information may also be interleaved with data in sectors stored on the raised areas.
Nano-imprint lithography processes may inevitably have some degree of eccentricity (e.g., 5 microns) between the DTR pattern center and the disk's geometric center resulting from less than perfect alignment procedures between a stamper and the disk during embossing operations. Such eccentricity may also result in problems with the positioning of a head over the DTR tracks during read and write operations of the disk drive. If the disk is mounted on the spindle hub using conventional assembly processes, the mass center of the disk would be aligned with the rotational center of the spindle without consideration as to the eccentricity of the DTR pattern center.
The basis for conventional assembly processes is that the mass center may be at point A, while the rotational center (substantially the ID geometric center at which the disk rotates when mounted) may be at a different point B, as illustrated in FIG. 1. A disk having this type of misalignment may encounter balancing problems when rotated on the spindle of a disk drive system. In order to ensure proper rotation of a disk on a spindle, conventional assembly processes attempt to locate the mass center of the disk at the rotational center at which the disk rotates. Thus, a mass balanced disk is one in which the mass center of the disk equates to the rotational center of the disk. A mass balanced disk is functionally important, because newer disk drive systems require higher rotational speeds. A high rotational speed of an unbalanced disk may lead to poor performance or disk failure. In addition, proper balancing is also necessary to achieve high track density by enabling the read/write head to accurately follow data tracks on a disk. However, conventional assembly processes do not account for the eccentricity of the DTR pattern center relative to the rotational center of the spindle.
One paper by Ho Seong Lee and Herman Ferrier, entitled “Open-Loop Compensation Of Repeatable Runouts In Discrete-Track Magnetic Disks,” ISPS-Vol. 1, ASME, 1995 analyzed run-outs due to DTR pattern eccentricity. This paper discusses experiments where a disk was installed on an adjustable spindle chuck under conditions where a repeatable run-out (RRO) was observed. In order to minimize the observed run-outs, the center of the spindle chuck was visually adjusted through a video monitor to adjust the run-out.
One problem with such an analysis is that it does not address non-repeatable run-out due to the disk's weight imbalance when the disk's mass center does not coincide with the rotational center of the spindle. During disk drive operation, such non-repeatable run-out of the disk results from the spindle vibration excited by the moment of the rotating disk. The predefined DTR track pattern on the disk must be followed by a head in order to read and write on the data tracks. To do so, the head must be moved constantly to compensate for such off-track deviations of the head due to non-repeatable run-out.
U.S. Pat. No. 5,907,448 describes a technique that servo-tracks a head where eccentricity exists between the rotational center of the disk and the DTR tracks' center. A suspension arm that supports the head in the disk drive needs to be moved constantly to follow the DTR tracks to read and write on a particular data track, if the track is not concentric with the rotational center of the disk. The suspension arm is electronically controlled to follow the DTR tracks that are, in effect, moving relative to the rotational path around the center of the disk by a sinusoidal input to a voice coil motor that drives the suspension arm. One problem with such an active, electronic control solution is that it does not minimize the eccentricity of the system when the disk is assembled into the disk drive. Such a solution consumes power even in the track following mode and adds complexity to the servo scheme in order to accommodate the burden of off-track head deviations.